Automated methods and systems for identifying and assigning attributes to human-face-containing subimages of input images

ABSTRACT

The present document is directed to methods and systems that identify and characterize subimages in images that each includes an image of a human face. In certain implementations, values for attributes, such as age, ethnicity, and gender, are assigned to face-containing subimages by the currently disclosed methods and systems. In these implementations, multiple feature-extractors output feature vectors that are distributed among attribute classifiers which consist of individual classifiers and, more often, multiple individual classifiers within aggregate classifiers. Attribute classifiers return indications of attribute values along with a probability value. Attribute classifiers are ordered with respect to reliability and applied in reliability order to generate attribute-assignment paths through a logical attribute-assignment tree, with uncertain attribute assignments generating multiple lower-level pathways.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application No. 62/214,785, filed Sep. 4, 2015, which is herein incorporated by reference.

TECHNICAL FIELD

The present document is related to image processing and facial-image characterization and, in particular, to methods and systems that identify and characterize document subimages in images that each contain an image of a human face.

BACKGROUND

While, for many years, computer scientists assumed that many complex tasks carried about by humans, including recognition and characterization of objects in images, would be rapidly automated by various techniques and approaches that were referred to as “artificial intelligence” (“AI”), the optimistic forecasts for optimization were not, in most cases, reflected in actual technical and scientific developments and progress. Many seemingly tractable computational problems proved to be far more complex than originally imagined and the hardware platforms, despite rapid evolution in capabilities and capacities, fell short of the computational bandwidths needed for automation of the complex tasks.

During the past 10 years, significant advances in distributed computing, including the emergence of cloud computing, have placed enormous computational bandwidth at the disposal of computational-bandwidth consumers, and is now routinely used for data analytics, scientific computation, web-site hosting, and for carrying out AI computations. However, even with the computational-bandwidth constraints relieved by massive distributed-computing systems, many problems remain difficult. Currently, designers and developers of systems that automate tasks formerly assumed to require human intelligence, including face recognition and identification of objects in images, continue to seek methods and subsystems that effectively harness the available computational bandwidth to address complex problem domains.

SUMMARY

The present document is directed to methods and systems that identify and characterize subimages in images that each includes an image of a human face. In certain implementations, values for attributes, such as age, ethnicity, and gender, are assigned to face-containing subimages by the currently disclosed methods and systems. In these implementations, multiple feature-extractors output feature vectors that are distributed among attribute classifiers which consist of individual classifiers and, more often, multiple individual classifiers within aggregate classifiers. Attribute classifiers return indications of attribute values along with a probability value. Attribute classifiers are ordered with respect to reliability and applied in reliability order to generate attribute-assignment paths through a logical attribute-assignment tree, with uncertain attribute assignments generating multiple lower-level pathways. By using this attribute-assignment method to efficiently traverse an otherwise generally imposing computational complexity, rapid, real-time or near-real time processing of images to assign attribute values to identified subimages of human faces is provided by the methods and systems to which the present document is directed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a general architectural diagram for various types of computers.

FIG. 2 illustrates an Internet-connected distributed computer system.

FIG. 3 illustrates cloud computing.

FIG. 4 illustrates generalized hardware and software components of a general-purpose computer system, such as a general-purpose computer system having an architecture similar to that shown in FIG. 1.

FIGS. 5A-D illustrate several types of virtual machine and virtual-machine execution environments.

FIG. 6 illustrates virtual data centers provided as an abstraction of underlying physical-data-center hardware components.

FIG. 7 illustrates a typical digitally encoded image.

FIG. 8 illustrates one version of the RGB color model.

FIG. 9 shows a different color model, referred to as the “hue-saturation-lightness” (“HSL”) color model.

FIG. 10 illustrates generation of a grayscale or binary image from a color image.

FIGS. 11A-F illustrate one approach to mapping points in a world coordinate system to corresponding points on an image plane of a camera.

FIG. 12 illustrates feature detection by the SIFT technique.

FIGS. 13-18 provide background information for various concepts used by the SIFT technique to identify features within images.

FIGS. 19A-D illustrate the selection of candidate feature points from an image.

FIG. 19E illustrates filtering of candidate keypoints, or features, in the difference-of-Gaussian layers generated by the SIFT technique.

FIG. 19F illustrates how the magnitude and orientation of a feature is assigned from values in a difference-of-Gaussian layer.

FIG. 190 illustrates computation of a descriptor for a feature.

FIGS. 19H-I illustrate a simple, one-parameter application of the Hough transform.

FIGS. 19J-K illustrate use of SIFT points to recognize objects in images.

FIG. 20 illustrates a second type of feature detector, referred to as the “Histogram of Gradients” (“HoG”) feature detector.

FIGS. 21A-B illustrate a third type of feature detector, referred to as the “Linear Binary Patterns” (“LBP”) feature detector.

FIG. 22 illustrates use of feature detectors in the methods and systems to which the current document is directed.

FIGS. 23A-B illustrate a type of classifier referred to as a support vector machine (“SVM”).

FIG. 24 illustrates two additional, higher-level feature detectors used in the methods and systems to which the current document is directed.

FIG. 25 illustrates normalization of the regions obtained by application of a face detector and face-subregions detector, discussed above with reference to FIG. 24.

FIG. 26 illustrates attribute classifiers employed in the methods and systems to which the current application is directed.

FIG. 27 illustrates the high-level architecture for the attribute-assignment image-processing system to which the current document is directed.

FIGS. 28A-D illustrate how aggregate classifiers produce output values and associated probabilities.

FIG. 29 illustrates a confusion matrix. The confusion matrix is obtained by observing the attribute values returned by a classifier for a number of input feature vectors with known attribute values.

FIG. 30 illustrates the high-level operation of the attribute-assigning image-processing system to which the current document is directed.

FIG. 31 illustrates one physical implementation of the attribute-assigning image-processing system to which the current document is directed.

FIGS. 32-34 illustrate an efficient attribute-assignment method used in many implementations of the attribute-assigning image-processing system to which the current document is directed.

FIGS. 35A-D illustrate one implementation of controller 3114 discussed with reference to FIG. 31.

DETAILED DESCRIPTION

The current document is directed to methods and systems that identify and characterize subimages containing images of human faces within an input image. The following discussion is subdivided into a number of subsections, including: (1) An Overview of Computer Systems and Architecture; (2) An Overview of Digital Images; (3) Perspective Transformations; (4) Feature Detectors; and (5) a final subsection in which the methods and systems to which the current document is directed are discussed.

Overview of Computer Systems and Computer Architecture

FIG. 1 provides a general architectural diagram for various types of computers. The computer system contains one or multiple central processing units (“CPUs”) 102-105, one or more electronic memories 108 interconnected with the CPUs by a CPU/memory-subsystem bus 110 or multiple busses, a first bridge 112 that interconnects the CPU/memory-subsystem bus 110 with additional busses 114 and 116, or other types of high-speed interconnection media, including multiple, high-speed serial interconnects. These busses or serial interconnections, in turn, connect the CPUs and memory with specialized processors, such as a graphics processor 118, and with one or more additional bridges 120, which are interconnected with high-speed serial links or with multiple controllers 122-127, such as controller 127, that provide access to various different types of mass-storage devices 128, electronic displays, input devices, and other such components, subcomponents, and computational resources. It should be noted that computer-readable data-storage devices include optical and electromagnetic disks, electronic memories, and other physical data-storage devices. Those familiar with modern science and technology appreciate that electromagnetic radiation and propagating signals do not store data for subsequent retrieval, and can transiently “store” only a byte or less of information per mile, far less information than needed to encode even the simplest of routines.

Of course, there are many different types of computer-system architectures that differ from one another in the number of different memories, including different types of hierarchical cache memories, the number of processors and the connectivity of the processors with other system components, the number of internal communications busses and serial links, and in many other ways. However, computer systems generally execute stored programs by fetching instructions from memory and executing the instructions in one or more processors. Computer systems include general-purpose computer systems, such as personal computers (“PCs”), various types of servers and workstations, and higher-end mainframe computers, but may also include a plethora of various types of special-purpose computing devices, including data-storage systems, communications routers, network nodes, tablet computers, and mobile telephones.

FIG. 2 illustrates an Internet-connected distributed computer system. As communications and networking technologies have evolved in capability and accessibility, and as the computational bandwidths, data-storage capacities, and other capabilities and capacities of various types of computer systems have steadily and rapidly increased, much of modern computing now generally involves large distributed systems and computers interconnected by local networks, wide-area networks, wireless communications, and the Internet. FIG. 2 shows a typical distributed system in which a large number of PCs 202-205, a high-end distributed mainframe system 210 with a large data-storage system 212, and a large computer center 214 with large numbers of rack-mounted servers or blade servers all interconnected through various communications and networking systems that together comprise the Internet 216. Such distributed computer systems provide diverse arrays of functionalities. For example, a PC user sitting in a home office may access hundreds of millions of different web sites provided by hundreds of thousands of different web servers throughout the world and may access high-computational-bandwidth computing services from remote computer facilities for running complex computational tasks.

Until recently, computational services were generally provided by computer systems and data centers purchased, configured, managed, and maintained by service-provider organizations. For example, an e-commerce retailer generally purchased, configured, managed, and maintained a data center including numerous web servers, back-end computer systems, and data-storage systems for serving web pages to remote customers, receiving orders through the web-page interface, processing the orders, tracking completed orders, and other myriad different tasks associated with an e-commerce enterprise.

FIG. 3 illustrates cloud computing. In the recently developed cloud-computing paradigm, computing cycles and data-storage facilities are provided to organizations and individuals by cloud-computing providers. In addition, larger organizations may elect to establish private cloud-computing facilities in addition to, or instead of, subscribing to computing services provided by public cloud-computing service providers. In FIG. 3, a system administrator for an organization, using a PC 302, accesses the organization's private cloud 304 through a local network 306 and private-cloud interface 308 and also accesses, through the Internet 310, a public cloud 312 through a public-cloud services interface 314. The administrator can, in either the case of the private cloud 304 or public cloud 312, configure virtual computer systems and even entire virtual data centers and launch execution of application programs on the virtual computer systems and virtual data centers in order to carry out any of many different types of computational tasks. As one example, a small organization may configure and run a virtual data center within a public cloud that executes web servers to provide an e-commerce interface through the public cloud to remote customers of the organization, such as a user viewing the organization's e-commerce web pages on a remote user system 316.

FIG. 4 illustrates generalized hardware and software components of a general-purpose computer system, such as a general-purpose computer system having an architecture similar to that shown in FIG. 1. The computer system 400 is often considered to include three fundamental layers: (1) a hardware layer or level 402; (2) an operating-system layer or level 404; and (3) an application-program layer or level 406. The hardware layer 402 includes one or more processors 408, system memory 410, various different types of input-output (“I/O”) devices 410 and 412, and mass-storage devices 414. Of course, the hardware level also includes many other components, including power supplies, internal communications links and busses, specialized integrated circuits, many different types of processor-controlled or microprocessor-controlled peripheral devices and controllers, and many other components. The operating system 404 interfaces to the hardware level 402 through a low-level operating system and hardware interface 416 generally comprising a set of non-privileged computer instructions 418, a set of privileged computer instructions 420, a set of non-privileged registers and memory addresses 422, and a set of privileged registers and memory addresses 424. In general, the operating system exposes non-privileged instructions, non-privileged registers, and non-privileged memory addresses 426 and a system-call interface 428 as an operating-system interface 430 to application programs 432-436 that execute within an execution environment provided to the application programs by the operating system. The operating system, alone, accesses the privileged instructions, privileged registers, and privileged memory addresses. By reserving access to privileged instructions, privileged registers, and privileged memory addresses, the operating system can ensure that application programs and other higher-level computational entities cannot interfere with one another's execution and cannot change the overall state of the computer system in ways that could deleteriously impact system operation. The operating system includes many internal components and modules, including a scheduler 442, memory management 444, a file system 446, device drivers 448, and many other components and modules. To a certain degree, modern operating systems provide numerous levels of abstraction above the hardware level, including virtual memory, which provides to each application program and other computational entities a separate, large, linear memory-address space that is mapped by the operating system to various electronic memories and mass-storage devices. The scheduler orchestrates interleaved execution of various different application programs and higher-level computational entities, providing to each application program a virtual, stand-alone system devoted entirely to the application program. From the application program's standpoint, the application program executes continuously without concern for the need to share processor resources and other system resources with other application programs and higher-level computational entities. The device drivers abstract details of hardware-component operation, allowing application programs to employ the system-call interface for transmitting and receiving data to and from communications networks, mass-storage devices, and other I/O devices and subsystems. The file system 446 facilitates abstraction of mass-storage-device and memory resources as a high-level, easy-to-access, file-system interface. In many modern operating systems, the operating system provides an execution environment for concurrent execution of a large number of processes, each corresponding to an executing application program, on one or a relatively small number of hardware processors by temporal multiplexing of process execution. Thus, the development and evolution of the operating system has resulted in the generation of a type of multi-faceted virtual execution environment for application programs and other higher-level computational entities.

While the execution environments provided by operating systems have proved to be an enormously successful level of abstraction within computer systems, the operating-system-provided level of abstraction is nonetheless associated with difficulties and challenges for developers and users of application programs and other higher-level computational entities. One difficulty arises from the fact that there are many different operating systems that run within various different types of computer hardware. In many cases, popular application programs and computational systems are developed to run on only a subset of the available operating systems, and can therefore be executed within only a subset of the various different types of computer systems on which the operating systems are designed to run. Often, even when an application program or other computational system is ported to additional operating systems, the application program or other computational system can nonetheless run more efficiently on the operating systems for which the application program or other computational system was originally targeted. Another difficulty arises from the increasingly distributed nature of computer systems. Although distributed operating systems are the subject of considerable research and development efforts, many of the popular operating systems are designed primarily for execution on a single computer system. In many cases, it is difficult to move application programs, in real time, between the different computer systems of a distributed computer system for high-availability, fault-tolerance, and load-balancing purposes. The problems are even greater in heterogeneous distributed computer systems which include different types of hardware and devices running different types of operating systems. Operating systems continue to evolve, as a result of which certain older application programs and other computational entities may be incompatible with more recent versions of operating systems for which they are targeted, creating compatibility issues that are particularly difficult to manage in large distributed systems.

For all of these reasons, a higher level of abstraction, referred to as the “virtual machine,” has been developed and evolved to further abstract computer hardware in order to address many difficulties and challenges associated with traditional computing systems, including the compatibility issues discussed above. FIGS. 5A-D illustrate several types of virtual machine and virtual-machine execution environments. FIGS. 5A-D use the same illustration conventions as used in FIG. 4. FIG. 5A shows a first type of virtualization. The computer system 500 in FIG. 5A includes the same hardware layer 502 as the hardware layer 402 shown in FIG. 4. However, rather than providing an operating system layer directly above the hardware layer, as in FIG. 4, the virtualized computing environment illustrated in FIG. 5A features a virtualization layer 504 that interfaces through a virtualization-layer/hardware-layer interface 506, equivalent to interface 416 in FIG. 4, to the hardware. The virtualization layer provides a hardware-like interface 508 to a number of virtual machines, such as virtual machine 510, executing above the virtualization layer in a virtual-machine layer 512. Each virtual machine includes one or more application programs or other higher-level computational entities packaged together with an operating system, referred to as a “guest operating system,” such as application 514 and guest operating system 516 packaged together within virtual machine 510. Each virtual machine is thus equivalent to the operating-system layer 404 and application-program layer 406 in the general-purpose computer system shown in FIG. 4. Each guest operating system within a virtual machine interfaces to the virtualization-layer interface 508 rather than to the actual hardware interface 506. The virtualization layer partitions hardware resources into abstract virtual-hardware layers to which each guest operating system within a virtual machine interfaces. The guest operating systems within the virtual machines, in general, are unaware of the virtualization layer and operate as if they were directly accessing a true hardware interface. The virtualization layer ensures that each of the virtual machines currently executing within the virtual environment receive a fair allocation of underlying hardware resources and that all virtual machines receive sufficient resources to progress in execution. The virtualization-layer interface 508 may differ for different guest operating systems. For example, the virtualization layer is generally able to provide virtual hardware interfaces for a variety of different types of computer hardware. This allows, as one example, a virtual machine that includes a guest operating system designed for a particular computer architecture to run on hardware of a different architecture. The number of virtual machines need not be equal to the number of physical processors or even a multiple of the number of processors.

The virtualization layer includes a virtual-machine-monitor module 518 (“VMM”) that virtualizes physical processors in the hardware layer to create virtual processors on which each of the virtual machines executes. For execution efficiency, the virtualization layer attempts to allow virtual machines to directly execute non-privileged instructions and to directly access non-privileged registers and memory. However, when the guest operating system within a virtual machine accesses virtual privileged instructions, virtual privileged registers, and virtual privileged memory through the virtualization-layer interface 508, the accesses result in execution of virtualization-layer code to simulate or emulate the privileged resources. The virtualization layer additionally includes a kernel module 520 that manages memory, communications, and data-storage machine resources on behalf of executing virtual machines (“VM kernel”). The VM kernel, for example, maintains shadow page tables on each virtual machine so that hardware-level virtual-memory facilities can be used to process memory accesses. The VM kernel additionally includes routines that implement virtual communications and data-storage devices as well as device drivers that directly control the operation of underlying hardware communications and data-storage devices. Similarly, the VM kernel virtualizes various other types of I/O devices, including keyboards, optical-disk drives, and other such devices. The virtualization layer essentially schedules execution of virtual machines much like an operating system schedules execution of application programs, so that the virtual machines each execute within a complete and fully functional virtual hardware layer.

FIG. 5B illustrates a second type of virtualization. In FIG. 5B, the computer system 540 includes the same hardware layer 542 and software layer 544 as the hardware layer 402 shown in FIG. 4. Several application programs 546 and 548 are shown running in the execution environment provided by the operating system. In addition, a virtualization layer 550 is also provided, in computer 540, but, unlike the virtualization layer 504 discussed with reference to FIG. 5A, virtualization layer 550 is layered above the operating system 544, referred to as the “host OS,” and uses the operating system interface to access operating-system-provided functionality as well as the hardware. The virtualization layer 550 comprises primarily a VMM and a hardware-like interface 552, similar to hardware-like interface 508 in FIG. 5A. The virtualization-layer/hardware-layer interface 552, equivalent to interface 416 in FIG. 4, provides an execution environment for a number of virtual machines 556-558, each including one or more application programs or other higher-level computational entities packaged together with a guest operating system.

While the traditional virtual-machine-based virtualization layers, described with reference to FIGS. 5A-B, have enjoyed widespread adoption and use in a variety of different environments, from personal computers to enormous distributed computing systems, traditional virtualization technologies are associated with computational overheads. While these computational overheads have been steadily decreased, over the years, and often represent ten percent or less of the total computational bandwidth consumed by an application running in a virtualized environment, traditional virtualization technologies nonetheless involve computational costs in return for the power and flexibility that they provide. Another approach to virtualization is referred to as operating-system-level virtualization (“OSL virtualization”). FIG. 5C illustrates the OSL-virtualization approach. In FIG. 5C, as in previously discussed FIG. 4, an operating system 404 runs above the hardware 402 of a host computer. The operating system provides an interface for higher-level computational entities, the interface including a system-call interface 428 and exposure to the non-privileged instructions and memory addresses and registers 426 of the hardware layer 402. However, unlike in FIG. 5A, rather than applications running directly above the operating system, OSL virtualization involves an OS-level virtualization layer 560 that provides an operating-system interface 562-564 to each of one or more containers 566-568. The containers, in turn, provide an execution environment for one or more applications, such as application 570 running within the execution environment provided by container 566. The container can be thought of as a partition of the resources generally available to higher-level computational entities through the operating system interface 430. While a traditional virtualization layer can simulate the hardware interface expected by any of many different operating systems, OSL virtualization essentially provides a secure partition of the execution environment provided by a particular operating system. As one example, OSL virtualization provides a file system to each container, but the file system provided to the container is essentially a view of a partition of the general file system provided by the underlying operating system. In essence, OSL virtualization uses operating-system features, such as name space support, to isolate each container from the remaining containers so that the applications executing within the execution environment provided by a container are isolated from applications executing within the execution environments provided by all other containers. As a result, a container can be booted up much faster than a virtual machine, since the container uses operating-system-kernel features that are already available within the host computer. Furthermore, the containers share computational bandwidth, memory, network bandwidth, and other computational resources provided by the operating system, without resource overhead allocated to virtual machines and virtualization layers. Again, however, OSL virtualization does not provide many desirable features of traditional virtualization. As mentioned above, OSL virtualization does not provide a way to run different types of operating systems for different groups of containers within the same host system, nor does OSL-virtualization provide for live migration of containers between host computers, as does traditional virtualization technologies.

FIG. 5D illustrates an approach to combining the power and flexibility of traditional virtualization with the advantages of OSL virtualization. FIG. 5D shows a host computer similar to that shown in FIG. 5A, discussed above. The host computer includes a hardware layer 502 and a virtualization layer 504 that provides a simulated hardware interface 508 to an operating system 572. Unlike in FIG. 5A, the operating system interfaces to an OSL-virtualization layer 574 that provides container execution environments 576-578 to multiple application programs. Running containers above a guest operating system within a virtualized host computer provides many of the advantages of traditional virtualization and OSL virtualization. Containers can be quickly booted in order to provide additional execution environments and associated resources to new applications. The resources available to the guest operating system are efficiently partitioned among the containers provided by the OSL-virtualization layer 574. Many of the powerful and flexible features of the traditional virtualization technology can be applied to containers running above guest operating systems including live migration from one host computer to another, various types of high-availability and distributed resource sharing, and other such features. Containers provide share-based allocation of computational resources to groups of applications with guaranteed isolation of applications in one container from applications in the remaining containers executing above a guest operating system. Moreover, resource allocation can be modified at run time between containers. The traditional virtualization layer provides flexible and easy scaling and a simple approach to operating-system upgrades and patches. Thus, the use of OSL virtualization above traditional virtualization, as illustrated in FIG. 5D, provides much of the advantages of both a traditional virtualization layer and the advantages of OSL virtualization. Note that, although only a single guest operating system and OSL virtualization layer as shown in FIG. 5D, a single virtualized host system can run multiple different guest operating systems within multiple virtual machines, each of which supports one or more containers.

In FIGS. 5A-D, the layers are somewhat simplified for clarity of illustration. For example, portions of the virtualization layer 550 may reside within the host-operating-system kernel, such as a specialized driver incorporated into the host operating system to facilitate hardware access by the virtualization layer.

It should be noted that virtual hardware layers, virtualization layers, operating systems, containers, and computer-instruction implemented systems that execute within execution environments provided by virtualization layers, operating systems, and containers are all physical entities that include electromechanical components and computer instructions stored in physical data-storage devices, including electronic memories, mass-storage devices, optical disks, magnetic disks, and other such devices. The term “virtual” does not, in any way, imply that virtual hardware layers, virtualization layers, and guest operating systems are abstract or intangible. Virtual hardware layers, virtualization layers, operating systems, containers, and higher-level systems execute on physical processors of physical computer systems and control operation of the physical computer systems, including operations that alter the physical states of physical devices, including electronic memories and mass-storage devices. They are as physical and tangible as any other component of a computer since, such as power supplies, controllers, processors, busses, and data-storage devices.

The advent of virtual machines and virtual environments has alleviated many of the difficulties and challenges associated with traditional general-purpose computing. Machine and operating-system dependencies can be significantly reduced or entirely eliminated by packaging applications and operating systems together as virtual machines and virtual appliances that execute within virtual environments provided by virtualization layers running on many different types of computer hardware. A next level of abstraction, referred to as virtual data centers which are one example of a broader virtual-infrastructure category, provide a data-center interface to virtual data centers computationally constructed within physical data centers. FIG. 6 illustrates virtual data centers provided as an abstraction of underlying physical-data-center hardware components. In FIG. 6, a physical data center 602 is shown below a virtual-interface plane 604. The physical data center consists of a virtual-infrastructure management server (“VI-management-server”) 606 and any of various different computers, such as PCs 608, on which a virtual-data-center management interface may be displayed to system administrators and other users. The physical data center additionally includes generally large numbers of server computers, such as server computer 610, that are coupled together by local area networks, such as local area network 612 that directly interconnects server computer 610 and 614-620 and a mass-storage array 622. The physical data center shown in FIG. 6 includes three local area networks 612, 624, and 626 that each directly interconnects a bank of eight servers and a mass-storage array. The individual server computers, such as server computer 610, each includes a virtualization layer and runs multiple virtual machines. Different physical data centers may include many different types of computers, networks, data-storage systems and devices connected according to many different types of connection topologies. The virtual-data-center abstraction layer 604, a logical abstraction layer shown by a plane in FIG. 6, abstracts the physical data center to a virtual data center comprising one or more resource pools, such as resource pools 630-632, one or more virtual data stores, such as virtual data stores 634-636, and one or more virtual networks. In certain implementations, the resource pools abstract banks of physical servers directly interconnected by a local area network.

The virtual-data-center management interface allows provisioning and launching of virtual machines with respect to resource pools, virtual data stores, and virtual networks, so that virtual-data-center administrators need not be concerned with the identities of physical-data-center components used to execute particular virtual machines. Furthermore, the VI-management-server includes functionality to migrate running virtual machines from one physical server to another in order to optimally or near optimally manage resource allocation, provide fault tolerance, and high availability by migrating virtual machines to most effectively utilize underlying physical hardware resources, to replace virtual machines disabled by physical hardware problems and failures, and to ensure that multiple virtual machines supporting a high-availability virtual appliance are executing on multiple physical computer systems so that the services provided by the virtual appliance are continuously accessible, even when one of the multiple virtual appliances becomes compute bound, data-access bound, suspends execution, or fails. Thus, the virtual data center layer of abstraction provides a virtual-data-center abstraction of physical data centers to simplify provisioning, launching, and maintenance of virtual machines and virtual appliances as well as to provide high-level, distributed functionalities that involve pooling the resources of individual physical servers and migrating virtual machines among physical servers to achieve load balancing, fault tolerance, and high availability.

An Overview of Digital Images

FIG. 7 illustrates a typical digitally encoded image. The encoded image comprises a two dimensional array of pixels 702. In FIG. 7, each small square, such as square 704, is a pixel, generally defined as the smallest-granularity portion of an image that is numerically specified in the digital encoding. Each pixel is a location, generally represented as a pair of numeric values corresponding to orthogonal x and y axes 706 and 708, respectively. Thus, for example, pixel 704 has x, y coordinates (39,0), while pixel 712 has coordinates (0,0). In the digital encoding, the pixel is represented by numeric values that specify how the region of the image corresponding to the pixel is to be rendered upon printing, display on a computer screen, or other display. Commonly, for black-and-white images, a single numeric value range of 0-255 is used to represent each pixel, with the numeric value corresponding to the grayscale level at which the pixel is to be rendered. In a common convention, the value “0” represents black and the value “255” represents white. For color images, any of a variety of different color-specifying sets of numeric values may be employed. In one common color model, as shown in FIG. 4, each pixel is associated with three values, or coordinates (r,g,b), which specify the red, green, and blue intensity components of the color to be displayed in the region corresponding to the pixel.

FIG. 8 illustrates one version of the RGB color model. The entire spectrum of colors is represented, as discussed above with reference to FIG. 3, by a three-primary-color coordinate (r,g,b). The color model can be considered to correspond to points within a unit cube 802 within a three-dimensional color space defined by three orthogonal axes: (1) r 804; (2) g 806; and (3) b 808. Thus, the individual color coordinates range from 0 to 1 along each of the three color axes. The pure blue color, for example, of greatest possible intensity corresponds to the point 810 on the b axis with coordinates (0,0,1). The color white corresponds to the point 812, with coordinates (1,1,1,) and the color black corresponds to the point 814, the origin of the coordinate system, with coordinates (0,0,0).

FIG. 9 shows a different color model, referred to as the “hue-saturation-lightness” (“HSL”) color model. In this color model, colors are contained within a three-dimensional bi-pyramidal prism 900 with a hexagonal cross section. Hue (h) is related to the dominant wavelength of a light radiation perceived by an observer. The value of the hue varies from 0° to 360° beginning with red 902 at 0°, passing through green 904 at 120°, blue 906 at 240°, and ending with red 902 at 360°. Saturation (s), which ranges from 0 to 1, is inversely related to the amount of white and black mixed with a particular wavelength, or hue. For example, the pure red color 902 is fully saturated, with saturation s=1.0, while the color pink has a saturation value less than 1.0 but greater than 0.0, white 908 is fully unsaturated, with s=0.0, and black 910 is also fully unsaturated, with s=0.0. Fully saturated colors fall on the perimeter of the middle hexagon that includes points 902, 904, and 906. A gray scale extends from black 910 to white 908 along the central vertical axis 912, representing fully unsaturated colors with no hue but different proportional combinations of black and white. For example, black 910 contains 100% of black and no white, white 908 contains 100% of white and no black and the origin 913 contains 50% of black and 50% of white. Lightness (l), or luma, represented by the central vertical axis 912, indicates the illumination level, ranging from 0 at black 910, with l=0.0, to 1 at white 908, with l=1.0. For an arbitrary color, represented in FIG. 9 by point 914, the hue is defined as angle θ 916, between a first vector from the origin 913 to point 902 and a second vector from the origin 913 to point 920 where a vertical line 922 that passes through point 914 intersects the plane 924 that includes the origin 913 and points 902, 904, and 906. The saturation is represented by the ratio of the distance of representative point 914 from the vertical axis 912, d′, divided by the length of a horizontal line passing through point 920 from the origin 913 to the surface of the bi-pyramidal prism 900, d. The lightness is the vertical distance from representative point 914 to the vertical level of the point representing black 910. The coordinates for a particular color in the HSL color model, (h,s,l), can be obtained from the coordinates of the color in the RGB color model, (r,g,b), as follows:

${l = \frac{\left( {C_{\max} - C_{\min}} \right)}{2}},{h = \begin{Bmatrix} {{{60{^\circ} \times \left( {\frac{g - b}{\Delta}{mod}{\;\;}6} \right)},}\mspace{14mu}} & {{{when}\mspace{14mu} C_{\max}} = r} \\ {{{60{^\circ} \times \left( {\frac{b - r}{\Delta} + 2} \right)},}\mspace{14mu}} & {{{when}\mspace{14mu} C_{\max}} = g} \\ {{{60{^\circ} \times \left( {\frac{r - g}{\Delta} + 4} \right)},}\mspace{14mu}} & {{{when}\mspace{14mu} C_{\max}} = b} \end{Bmatrix}},\mspace{14mu}{and}$ ${s = \begin{Bmatrix} {0,} & {\Delta = 0} \\ {\frac{\Delta}{1 - {{{2l} - 1}}},} & {otherwise} \end{Bmatrix}},$ where r, g, and b values are intensities of red, green, and blue primaries normalized to the range [0, 1]; C_(max) is a normalized intensity value equal to the maximum of r, g, and b; C_(min) is a normalized intensity value equal to the minimum of r, g, and b; and Δ is defined as C_(max)−C_(min).

FIG. 10 illustrates generation of a grayscale or binary image from a color image. In a color image, each pixel is generally associated with three values: a, b, and c 1002. Different color models employ different values of a, b, and c to represent a particular color. A grayscale image includes only a single intensity value 1004 for each pixel. A binary image is a special case of a grayscale image with only two different intensity values, 0 and 1. Commonly, grayscale images may have 256 or 65,536 different intensity values, with each pixel represented by a byte or 16-bit word, respectively. Thus, to transform a color image to grayscale, the three values a, b, and c in the color pixels need to be translated to single intensity values for the grayscale or binary image. In a first step, the three color values a, b, and c are transformed to a luminosity value L, generally in a range of [0.0, 1.0] 1006. For certain color models, a function is applied to each of the color values 1008 and the results are summed 1010 to produce the luminosity value. In other color models, each color value is multiplied by a coefficient and the results are summed 1012 to produce the luminosity value. In yet other color systems, one of the three color values is, in fact, the luminosity value 1014. Finally, in the general case, a function is applied to the three color values 1016 to produce the luminosity value. The luminosity value is then quantized 1018 to produce a grayscale intensity value within the desired range, generally [0, 255] for grayscale images and (0,1) for binary images.

Perspective Transformations

FIGS. 11A-F illustrate one approach to mapping points in a world coordinate system to corresponding points on an image plane of a camera. FIG. 11A illustrates the image plane of a camera, an aligned camera coordinate system and world coordinate system, and a point in three-dimensional space that is imaged on the image plane of the camera. In FIG. 11A, the camera coordinate system, comprising the x, y, and z axes, is aligned and coincident with the world-coordinate system X, Y, and Z. This is indicated, in FIG. 11A, by dual labeling of the x and X axis 1102, the y and Y axis 1104, and the z and Z axis 1106. The point that is imaged 1108 is shown to have the coordinates (X_(p), Y_(p), and Z_(p)). The image of this point on the camera image plane 1110 has the coordinates (x_(i), y_(i)). The virtual lens of the camera is centered at the point 1112, which has the camera coordinates (0, 0, l) and the world coordinates (0, 0, l). When the point 1108 is in focus, the distance l between the origin 1114 and point 1112 is the focal length of the camera. A small rectangle is shown, on the image plane, with the corners along one diagonal coincident with the origin 1114 and the point 1110 with coordinates (x_(i), y_(i)). The rectangle has horizontal sides, including horizontal side 1116, of length x_(i), and vertical sides, including vertical side 1118, with lengths y_(i). A corresponding rectangle with horizontal sides of length −X_(p), including horizontal side 1120, and vertical sides of length −Y_(p), including vertical side 1122. The point 1108 with world coordinates (−X_(p), −Y_(p), and Z_(p)) and the point 1124 with world coordinates (0, 0, Z_(p)) are located at the corners of one diagonal of the corresponding rectangle. Note that the positions of the two rectangles are inverted through point 1112. The length of the line segment 1128 between point 1112 and point 1124 is Z_(p)−l. The angles at which each of the lines shown in FIG. 11A passing through point 1112 intersects the z, Z axis are equal on both sides of point 1112. For example, angle 1130 and angle 1132 are identical. As a result, the principal of the correspondence between the lengths of similar sides of similar triangles can be used to derive expressions for the image-plane coordinates (x_(i), y_(i)) for an imaged point in three-dimensional space with world coordinates (X_(p), Y_(p), and Z_(p)) 1134:

$\frac{x_{i}}{l} = {\frac{- X_{p}}{Z_{p} - l} = {{\frac{X_{p}}{l - Z_{p}}\mspace{31mu}\frac{y_{i}}{l}} = {\frac{- Y_{p}}{Z_{p} - l} = \frac{Y_{p}}{l - Z_{p}}}}}$ $\;{{x_{i} = \frac{{IX}_{p}}{l - Z_{p}}},{y_{i} = \frac{{IY}_{p}}{l - Z_{p}}}}$

Camera coordinate systems are not, in general, aligned with the world coordinate system. Therefore, a slightly more complex analysis is required to develop the functions, or processes, that map points in three-dimensional space to points on the image plane of a camera.

FIG. 11B illustrates matrix equations that express various types of operations on points in a three-dimensional space. A translation 1134 a moves a first point with coordinates (x,y,z) 1134 b to a second point 1134 c with coordinates (x′,y′,z′). The translation involves displacements in the x 1134 d, y 1134 e, and z 1134 f directions. The matrix equation for the translation 1134 g is provided below the illustration of the translation 1134 a. Note that a fourth dimension is added to the vector representations of the points in order to express the translation as a matrix operation. The value “1” is used for the fourth dimension of the vectors and, following computation of the coordinates of the translated point, can be discarded. Similarly, a scaling operation 1134 h multiplies each coordinate of a vector by a scaling factor σ_(x), σ_(y), and σ_(z), respectively 1134 i, 1134 j, and 1134 k. The matrix equation for a scaling operation is provided by matrix equation 1134 l. Finally, a point may be rotated about each of the three coordinate axes. Diagram 1134 m shows rotation of a point (x,y,z) to the point (x′,y′,z′) by a rotation of γ radians about the z axis. The matrix equation for this rotation is shown as matrix equation 1134 n in FIG. 3B. Matrix equations 1134 o and 1134 p express rotations about the x and y axis, respectively, by α and β radians, respectively.

FIGS. 11C-E illustrate the process for computing the image of points in a three-dimensional space on the image plane of an arbitrarily oriented and positioned camera. FIG. 11C shows the arbitrarily positioned and oriented camera. The camera 1136 is mounted to a mount 1137 that allows the camera to be tilted by an angle α 1138 with respect to the vertical Z axis and to be rotated by an angle θ 1139 about a vertical axis. The mount 1137 can be positioned anywhere in three-dimensional space, with the position represented by a position vector w₀ 1140 from the origin of the world coordinate system 1141 to the mount 1137. A second vector r 1142 represents the relative position of the center of the image plane 1143 within the camera 1136 with respect to the mount 1137. The orientation and position of the origin of the camera coordinate system coincides with the center of the image plane 1143 within the camera 1136. The image plane 1143 lies within the x, y plane of the camera coordinate axes 1144-1146. The camera is shown, in FIG. 11C, imaging a point w 1147, with the image of the point w appearing as image point c 1148 on the image plane 1143 within the camera. The vector w₀ that defines the position of the camera mount 1137 is shown, in FIG. 11C, to be the vector

$w_{0} = {\begin{bmatrix} X_{0} \\ Y_{0} \\ Z_{0} \end{bmatrix}.}$

FIGS. 11D-E show the process by which the coordinates of a point in three-dimensional space, such as the point corresponding to vector w in world-coordinate-system coordinates, is mapped to the image plane of an arbitrarily positioned and oriented camera. First, a transformation between world coordinates and homogeneous coordinates h and the inverse transformation h⁻¹ is shown in FIG. 11D by the expressions 1150 and 1151. The forward transformation from world coordinates 1152 to homogeneous coordinates 1153 involves multiplying each of the coordinate components by an arbitrary constant k and adding a fourth coordinate component having the value k. The vector w corresponding to the point 1147 in three-dimensional space imaged by the camera is expressed as a column vector, as shown in expression 1154 in FIG. 11D. The corresponding column vector w_(k) in homogeneous coordinates is shown in expression 1155. The matrix P is the perspective transformation matrix, shown in expression 1156 in FIG. 11D. The perspective transformation matrix is used to carry out the world-to-camera coordinate transformations (1134 in FIG. 11A) discussed above with reference to FIG. 11A. The homogeneous-coordinate-form of the vector c corresponding to the image 1148 of point 1147, c_(h), is computed by the left-hand multiplication of w_(k) by the perspective transformation matrix, as shown in expression 1157 in FIG. 11D. Thus, the expression for c_(h) in homogeneous camera coordinates 1158 corresponds to the homogeneous expression for c_(h) in world coordinates 1159. The inverse homogeneous-coordinate transformation 1160 is used to transform the latter into a vector expression in world coordinates 1161 for the vector c 1162. Comparing the camera-coordinate expression 1163 for vector c with the world-coordinate expression for the same vector 1161 reveals that the camera coordinates are related to the world coordinates by the transformations (1134 in FIG. 11A) discussed above with reference to FIG. 11A. The inverse of the perspective transformation matrix, P⁻¹, is shown in expression 1164 in FIG. 11D. The inverse perspective transformation matrix can be used to compute the world-coordinate point in three-dimensional space corresponding to an image point expressed in camera coordinates, as indicated by expression 1166 in FIG. 11D. Note that, in general, the Z coordinate for the three-dimensional point imaged by the camera is not recovered by the perspective transformation. This is because all of the points in front of the camera along the line from the image point to the imaged point are mapped to the image point. Additional information is needed to determine the Z coordinate for three-dimensional points imaged by the camera, such as depth information obtained from a set of stereo images or depth information obtained by a separate depth sensor.

Three additional matrices are shown in FIG. 11E that represent the position and orientation of the camera in the world coordinate system. The translation matrix T_(w) _(o) 1170 represents the translation of the camera mount (1137 in FIG. 11C) from its position in three-dimensional space to the origin (1141 in FIG. 11C) of the world coordinate system. The matrix R represents the α and θ rotations needed to align the camera coordinate system with the world coordinate system 1172. The translation matrix C 1174 represents translation of the image plane of the camera from the camera mount (1137 in FIG. 11C) to the image plane's position within the camera represented by vector r (1142 in FIG. 11C). The full expression for transforming the vector for a point in three-dimensional space w_(h) into a vector that represents the position of the image point on the camera image plane c_(h) is provided as expression 1176 in FIG. 11E. The vector w_(h) is multiplied, from the left, first by the translation matrix 1170 to produce a first intermediate result, the first intermediate result is multiplied, from the left, by the matrix R to produce a second intermediate result, the second intermediate result is multiplied, from the left, by the matrix C to produce a third intermediate result, and the third intermediate result is multiplied, from the left, by the perspective transformation matrix P to produce the vector c_(h). Expression 1178 shows the inverse transformation. Thus, in general, there is a forward transformation from world-coordinate points to image points 1180 and, when sufficient information is available, an inverse transformation 1181. It is the forward transformation 1180 that is used to generate two-dimensional images from a three-dimensional model or object corresponding to arbitrarily oriented and positioned cameras. Each point on the surface of the three-dimensional object or model is transformed by forward transformation 1180 to points on the image plane of the camera.

FIG. 11F illustrates matrix equations that relate two different images of an object, when the two different images differ because of relative changes in the position, orientation, and distance from the camera of the objects, arising due to changes in the position and orientation of the camera, position and orientation of the objects being imaged, or both. Because multiplications of square matrices produce another square matrix, equation 1176 shown in FIG. 11E can be concisely expressed as equation 1190 in FIG. 11F. This equation determines the position of points in an image to the position of the corresponding points in a three-dimensional space. Equation 1191 represents computation of the points in a second image from corresponding points in a three-dimensional space where the points in the three-dimensional space have been altered in position or orientation from the corresponding points used to produce the points c_(h) in a first imaging operation represented by equation 1190. The T, R, and S matrices in equation 1191 represent translation, rotation, and scaling operations. Equation 1190 can be recast as equation 1192 by multiplying both sides of equation 1190 by the inverse of matrix M. Substituting the left side of equation 1192 into equation 1191 produces equation 1194, which relates positions in the first image, c_(h), to positions in the second image, e′_(h). Equation 1194 can be more succinctly represented as equation 1195 and alternatively as equation 1196. Because equation 1196 is expressing a relationship between positions of points in two images, and therefore the z coordinate is of no interest, equation 1196 can be recast as equation 1197 where the z-coordinate values are replaced by 0. Representing the matrix M* abstractly in equation 1198, a new matrix M** can be created by removing the third row and third column from matrix M*, as shown in equation 1199 a. Removing the z-coordinate values from the c_(h) and c′_(h) vectors, equation 1199 b is obtained. In the case that there are four pairs of points with known coordinates in each of the two images, the relationship between these four pairs of points can be expressed as equation 1199 c. This equation is slightly over-determined, but can be used to determine, by known techniques, values for the nine elements of the matrix M**. Thus, regardless of the differences in orientation, position, and distance from the camera of a set of objects during two different image-acquisition operation, a matrix can be determined, by comparing the positions of a number of known corresponding features in the two images, that represents the transformation and reverse transformation relating the two images.

Feature Detectors

Feature detectors are another type of image-processing methodology, various types of which are used in the methods and systems to which the current document is directed, as discussed below. A particular feature detector, referred to as the “Scale Invariant Feature Transform” (“SIFT”), is discussed in some detail, in the current subsection, as an example of the various feature detectors that may be employed in methods and systems to which the current document is directed.

FIG. 12 illustrates feature detection by the SIFT technique. In FIG. 12, a first simple digital image 1202 is shown to include a generally featureless background 1204 and a shaded disk region 1206. Application of SIFT feature detection to this image generates a set of keypoints or features, such as the features 1208-1217 overlaid on a copy 1220 of the original image, shown in FIG. 12 to the right of the original image. The features are essentially annotated points within the digital image, having coordinates (x,y) relative to image coordinate axes generally parallel to the top and left-hand edges of the image. These points are selected to be relatively invariant to image translation, scaling, and rotation and partially invariant to illumination changes and affine projection. Thus, in the case that a particular object is first imaged to generate a canonical image of the object, features generated by the SIFT technique for this first canonical image can be used to locate the object in additional images in which image acquisition differs in various ways, including perspective, illumination, location of the object relative to the camera, orientation of the object relative to the camera, or even physical distortion of the object. Each feature generated by the SIFT technique is encoded as a set of values and stored in a database, file, in-memory data structure, or other such data-storage entity. In FIG. 12, the stored descriptors are arranged in a table 1230, each row of which represents a different feature. Each row contains a number of different fields corresponding to columns in the table: (1) x 1231, the x coordinate of the feature; (2) y 1232, they coordinate of the feature; (3) m 1233, a magnitude value for the feature; (4) θ 1234, an orientation angle for the feature; (5) σ 1235, a scale value for the feature; and (6) a descriptor 1236, an encoded set of characteristics of the local environment of the feature that can be used to determine whether a local environment of a point in another image can be considered to be the same feature identified in the other image.

FIGS. 13-18 provide background information for various concepts used by the SIFT technique to identify features within images. FIG. 13 illustrates a discrete computation of an intensity gradient. In FIG. 13, a small square portion 1302 of a digital image is shown. Each cell, such as cell 1304, represents a pixel and the numeric value within the cell, such as the value “106” in cell 1304, represents a grayscale intensity. Consider pixel 1306 with the intensity value “203.” This pixel, and four contiguous neighbors, are shown in the cross-like diagram 1308 to the right of the portion 1302 of the digital image. Considering the left 1310 and right 1312 neighbor pixels, the change in intensity value in the x direction, Δx, can be discretely computed as:

${\Delta\; x} = {\frac{247 - 150}{2} = {48.5.}}$ Considering the lower 1314 and upper 1316 pixel neighbors, the change in intensity in the vertical direction, Δy, can be computed as:

${\Delta\; y} = {\frac{220 - 180}{2} = 20.}$ The computed Δx is an estimate of the partial differential of the continuous intensity function with respect to the x coordinate at the central pixel 1306:

${\frac{\partial F}{\partial x} \cong {\Delta\; x}} = {48.5.}$ The partial differential of the intensity function F with respect to the y coordinate at the central pixel 1306 is estimated by Δy:

${\frac{\partial F}{\partial y} \cong {\Delta\; y}} = 20.$ The intensity gradient at pixel 1306 can then be estimated as:

${gradient} = {{\nabla F} = {{{\frac{\partial F}{\partial x}i} + {\frac{\partial F}{\partial x}j}} = {{48.5i} + {20j}}}}$ where i and j are the unit vectors in the x and y directions. The magnitude of the gradient vector and the angle of the gradient vector are then computed as: |gradient|=√{square root over (48.5²+20²)}=52.5 θ=atan 2(20,48.5)=22.4 The direction of the intensity gradient vector 1320 and the angle θ 1322 are shown superimposed over the portion 1302 of the digital image in FIG. 13. Note that the gradient vector points in the direction of steepest increase in intensity from pixel 1306. The magnitude of the gradient vector indicates an expected increase in intensity per unit increment in the gradient direction. Of course, because the gradient is only estimated by discrete operations, in the computation illustrated in FIG. 13, both the direction and magnitude of the gradient are merely estimates.

FIG. 14 illustrates a gradient computed for a point on a continuous surface. FIG. 14 illustrates a continuous surface z=F(x,y). The continuous surface 1402 is plotted with respect to a three-dimensional Cartesian coordinate system 1404, and has a hat-like shape. Contour lines, such as contour line 1406, can be plotted on the surface to indicate a continuous set of points with a constant z value. At a particular point 1408 on a contour plotted on the surface, the gradient vector 1410 computed for the point is perpendicular to the contour line and points in the direction of the steepest increase along the surface from point 1408.

In general, an intensity gradient vector is oriented perpendicularly to an intensity edge, and the greater the magnitude of the gradient, the sharper the edge or the greatest difference in intensities of the pixels on either side of the edge. FIG. 15 illustrates a number of intensity-gradient examples. Each example, such as example 1502, includes a central pixel for which the gradient is computed and the four contiguous neighbors used to compute Δx and Δy. The sharpest intensity boundaries are shown in the first column 1504. In these cases, the magnitude of the gradient is at least 127.5 and, for the third case 1506, 180.3. A relatively small difference across an edge, shown in example 1508, produces a gradient with a magnitude of only 3.9. In all cases, the gradient vector is perpendicular to the apparent direction of the intensity edge through the central pixel.

Many image-processing methods involve application of kernels to the pixel grid that constitutes the image. FIG. 16 illustrates application of a kernel to an image. In FIG. 16, a small portion of an image 1602 is shown as a rectilinear grid of pixels. A small 3×3 kernel k 1604 is shown below the representation of image I 1602. A kernel is applied to each pixel of the image. In the case of a 3×3 kernel, such as kernel k 1604 shown in FIG. 16, a modified kernel may be used for edge pixels or the image can be expanded by copying the intensity values in edge pixels to a circumscribing rectangle of pixels so that the kernel can be applied to each pixel of the original image. To apply the kernel to an image pixel, the kernel 1604 is computationally layered over a neighborhood of the pixel to which the kernel is applied 1606 having the same dimensions, in pixels, as the kernel. Application of the kernel to the neighborhood of the pixel to which the kernel is applied produces a new value for the pixel in a transformed image produced by applying the kernel to pixels of the original image. In certain types of kernels, the new value for the pixel to which the kernel is applied, I_(n), is obtained as the sum of the products of the kernel value and pixel aligned with the kernel value 1608. In other cases, the new value for the pixel is a more complex function of the neighborhood about the pixel and the kernel 1610. In yet other types of image processing, a new value for a pixel is generated by a function applied to the neighborhood of the pixel, without using a kernel 1612.

FIG. 17 illustrates convolution of a kernel with an image. In general, the kernel is sequentially applied to each pixel of an image, in some cases, into each non-edge pixel of an image; in other cases, to produce new values for a transformed image. In FIG. 17, a 3×3 kernel, shown by shading 1702, is sequentially applied to the first row of non-edge pixels in an image 1704. Each new value generated by application of a kernel to a pixel in the original image 1706 is then placed into the transformed image 1707. In other words, the kernel is sequentially applied to the original neighborhoods of each pixel in the original image to produce the transformed image. This process is referred to as “convolution,” and is loosely related to the mathematical convolution operation computed by multiplying Fourier-transformed images and then carrying out an inverse Fourier transform on the product.

FIG. 18 illustrates some example kernel and kernel-like image-processing techniques. In the process referred to as “median filtering,” the intensity values in a neighborhood of the original image 1802 are sorted 1804 in ascending-magnitude order and the median value 1806 is selected as a new value 1808 for the corresponding neighborhood of the transformed image. Gaussian smoothing and denoising involves applying a Gaussian kernel 1810 to each neighborhood 1814 of the original image to produce the value for the central pixel of the neighborhood 1816 in the corresponding neighborhood of the processed image. The values in the Gaussian kernel are computed by an expression such as expression 1818 to produce a discrete representation of a Gaussian surface above the neighborhood formed by rotation of a bell-shaped curve about a vertical axis coincident with the central pixel. The horizontal and vertical components of the image gradient for each pixel can be obtained by application of the corresponding G_(x) 1820 and G_(y) 1822 gradient kernels. These are only three of the many different types of convolution-based image-processing techniques.

Returning to the SIFT technique, a first task is to locate candidate points in an image for designation as features. The candidate points are identified using a series of Gaussian filtering or smoothing and resampling steps to create a first Gaussian pyramid and then computing differences between adjacent layers in the first Gaussian pyramid to create a second difference-of-Gaussians (“DoG”) pyramid. Extrema points within neighborhoods of the DoG pyramid are selected as candidate features, with the maximum value of a point within the neighborhood used to determine a scale value for the candidate feature.

FIGS. 19A-D illustrate the selection of candidate feature points from an image. FIG. 19A shows changes in the shape of a one-dimensional Gaussian curve with increasing values of the variance parameter for the curve. A first one-dimensional Gaussian curve 1902 is obtained using an initial variance parameter 1903. By increasing the value of the variance parameter to produce a second variance parameter 1904, a broader Gaussian curve 1905 is obtained. As the variance parameter increases, the Gaussian curve becomes broader and broader, as can be seen in the series of Gaussian curves 1902, 1905, and 1906-1907. Convolution of a Gaussian kernel with an image generally removes fine-granularity detail and produces a smoothing of the image. As the variance parameter increases, the corresponding Gaussian kernel removes increasing levels of detail from an image, resulting in increasing blurring of the image, since the value assigned to the central pixel by a Gaussian kernel is the average of an increasing number of neighboring pixels.

FIG. 19B illustrates rescaling of an image. An initial image 1910 has ten rows of pixels and eight columns of pixels. The pixels in this image can be partitioned into 20 groups of four pixels each 1912, and the average value of the four pixels assigned to each group. These average values can then be reassembled into a rescaled image 1914 having fewer pixels. This is one, simplistic rescaling method. In general, rescaling can choose an arbitrary rescaling parameter with the values for the rescaled pixels computed from neighborhoods centered around grid points in the original image defined by the rescaling parameter.

FIG. 19C illustrates construction of the initial Gaussian pyramid and second DoG pyramid. The initial Gaussian pyramid 1920 begins with the original image 1922 as its lowest level. The original image is smoothed using a Gaussian kernel to produce a corresponding smoothed image 1924. The symbol “G” and vertical arrow 1925 represent the Gaussian smoothing operation. Next, the smooth image 1924 is rescaled to produce a rescaled image 1926. This rescaling operation is represented by the symbol “S” and arrow 1927. The rescaled image is then smoothed by application of a Gaussian filter to produce a smoothed rescale image 1928. These operations are repeated to create successive double layers within the initial Gaussian pyramid. The second, DoG pyramid 1930 is obtained by computing a difference image from each pair of adjacent images in the initial Gaussian pyramid 1920. For example, the first image 1932 in the second, DoG pyramid is obtained by computing the difference 1933 of the initial two images 1922 and 1924 in the initial Gaussian pyramid 1920. Because the Gaussian kernel is applied to ever-greater rescalings of the initial image, the smoothing operation represented by each successive Gaussian smoothing operation from bottom to the top of the Gaussian pyramid is effectively carrying out smoothing at greater values of the variance parameter. Thus, the images from bottom to top of the Gaussian pyramid 1920 are increasingly blurred. A scale value 1936-1939 is associated with each layer in the DoG pyramid 1930 reflective of the combined effects of rescaling and Gaussian smoothing, with a greater value of the scale parameter indicating increasing blurring of the image.

FIG. 19D illustrates selection of a candidate feature using the DoG pyramid obtained by the methods discussed above with reference to FIGS. 19A-C. A point in the lowest level of the DoG pyramid, such as point 1940, has corresponding locations in higher-level images 1941 and 1942 in the DoG pyramid 1930. A 3×3 neighborhood of the point in each of three successive layers 1944-1946 constitutes a neighborhood of the point within the DoG pyramid. If the value of the point is the maximum value in each neighborhood layer, then the point is selected as a candidate feature point. Furthermore, the scale parameter associated with the image from which the neighborhood layer is selected for the neighborhood layer in which the point has maximum value is selected as the scale for the candidate feature point (1235 in FIG. 12). Thus, by the method discussed with reference to FIGS. 19A-D, a set of candidate feature points is obtained along with the x, y, and σ parameters for the candidate feature point.

It should be noted that the above discussion is somewhat simplified. For example, the original image may be added as a lowest level of the DoG pyramid and minimization of a Taylor series may be used to more precisely locate the positions of points within layers of the Gaussian pyramid and DoG pyramids. In fact, this method is an approximation to generating a scale space as the Laplacian of Gaussians, a mathematical operation that lies at the foundation of a variety of different image-processing techniques.

FIG. 19E illustrates filtering of candidate keypoints, or features, in the DoG layers generated by the SIFT technique. Extreme values in these layers are candidate keypoints or features. Because the DoG procedure tends to emphasize points lying on edges, candidate keypoints or features are filtered to remove candidate keypoints or features close to edges as well as candidate keypoints or features with values less than a minimum or threshold value. Thus, of the initial set of candidate keypoints or features 1950 a, those candidate features with values less than the minimum or threshold value or with a ratio of principal curvatures greater than a value obtained using a different threshold r 1950 c are removed to produce a smaller, filtered set of candidate features 1950 d. The ratio of principal curvatures is obtained from the Hessian matrix 1950 e which describes the local curvature of a surface.

FIG. 19F illustrates how the magnitude and orientation of a feature is assigned from values in a DoG layer. A pixel neighborhood 1954 a about an identified candidate feature point 1954 b is considered. For each point in the neighborhood, such as point 1954 c, a gradient magnitude 1954 d and orientation angle 1954 e are estimated using differences in neighboring pixel values. A Gaussian weighting surface 1954 f is constructed above the local neighborhood 1954 a using the Gaussian parameter σ equal to a value v times the scale value for the DoG plane 1954 g. The height 1954 h of the weighting surface above a particular point, t, is used as a multiplier of the computed magnitude for the pixel to generate a weight w 1954 i. The weighted magnitudes of the pixels are then entered into a histogram 1954 j that includes 36 bins for each 10° range of the computed gradient orientations for the pixels. The magnitude and θ value for the highest peak produced 1954 k is used as the magnitude and θ value for the feature 1954 b. Any histogram peaks greater than or equal to 80 percent of the height of the highest peak, such as peaks 1954 l and 1954 m, are treated as additional features that are assigned the corresponding magnitudes and θ values of those peaks.

FIG. 19G illustrates computation of a descriptor for a feature. In FIG. 19G, a feature is located at position 1956 a within a 16×16 pixel neighborhood 1956 b. As with the Gaussian surface constructed as shown in FIG. 19F, a similar Gaussian surface is constructed above neighborhood 1956 b, as indicated by the dashed circle 1956 c in FIG. 19G. The height of the surface of a point above a point is used to weight gradient magnitudes determined for each pixel in neighborhood 1956 b. A histogram, such as histogram 1956 d, is constructed for each 4×4 neighborhood within neighborhood 1956 b, such as 4×4 neighborhood 1956 e. The weighted magnitudes for the pixels in each 4×4 neighborhood are placed into the histogram according to the computed gradient orientations for the pixels. In this case, the histogram has eight bins, each bin corresponding to a 45° range of θ values. The column heights, or magnitudes, of the histogram are then entered into a 128-element vector 1956 f to a form a feature descriptor. Determination of all of the attributes for SIFT features, shown as columns in table 1230 of FIG. 12, have now been described.

Another technique used both in identifying and characterizing SIFT features as well as in many other image-processing techniques is referred to as the Hough transform. FIGS. 19H-I illustrate a simple, one-parameter application of the Hough transform. It is the application for which the Hough transform was initially proposed in the image-processing context. As shown in FIG. 19H, the equations for lines in a two-dimensional plane can be expressed in a normal y=mx+b form 1958 a. In this left-hand plot, there are two lines: line 1 1958 b and line 2 1958 c. The y=mx+b equations for these lines are shown as expressions 1958 d and 1959 e, respectively. The lines can alternatively be represented in a r=x cos θ+y sin θ, as shown in the right-hand plot 1958 f in FIG. 19H. In this plot, the same two lines 1958 b and 1958 c are again plotted in a two-dimensional plane. The parameter r is the shortest distance between a line and the origin 1958 g of the coordinate axes that define coordinate assignments to points in the two-dimensional plane. The angle of the line segment that connects the line to the origin, such as line segment 1958 h, shown as a dashed line segment connecting line 1958 b to the origin 1958 g, with respect to the horizontal coordinate axis 1958 i is the parameter θ in the r=x cos θ+y sin θ form of the equations of the lines. The r=x cos θ+y sin θ forms for the expressions for the two lines 1958 b and 1958 c are shown as expressions 1958 j and 1958 k, below plot 1958 f.

FIG. 19I illustrates an example problem of determining whether or not a set of three data points plotted within a two-dimensional plane can be described as falling on a line and, if so, the equation for the line. Plot 1960 a, in the top right-hand corner of FIG. 19I, shows a plot of three data points 1960 b, 1960 c, and 1960 d, with their Cartesian coordinates shown next to the data points. These data points do not exactly fall on a line, but fall close to a line. In the Hough-Transform method, each data point places values in an r/θ parameter space corresponding to possible lines on which the data point is located. For example, data point 1960 b is shown in plot 1960 e as lying on seven different lines indicated by dashed-line segments, such as dashed-line segment 1960 f. These lines are equally spaced from one another in orientation by a relative orientation of 22.5° 1960 g. The r and θ parameters for each of these lines are computed from the shortest-distance line segments from each of the lines to the origin, such as shortest-distance line segment 1960 h connecting line 1960 i to the origin 1960 j. The r and θ values for each of the possible lines on which data point 1960 b is located are then tabulated in table 1960 k. The r and θ values for the lines on which data points 1960 c and 1960 d may reside are similarly tabulated in tables 19601 and 1960 m. These tabulated values may be considered as votes by each data point for particular r/θ pairs. The r/θ parameter space can be considered to be a grid of rectilinear cells, each cell representing a small range of r and θ values. The cell receiving the most votes, indicated in FIG. 19I by asterisks next to the votes, such as asterisk 1960 n, is selected as the r and θ value for the best line coincident with the positions of the data points. This line 1960 o is shown plotted with respect to the data points in plot 1960 p in the lower right-hand portion of FIG. 19I. As can be seen, the data points lie very close to this line. Thus, the Hough Transform can be described as a parameter-voting method by which data points vote for cells within a parameter space to which they may correspond, and cells with the greatest number of votes are selected as candidate sets of parameter values for a mathematical model of the relative position of the data points. The Hough-Transform technique is generally applicable to selecting mathematical models with an arbitrary number of parameters.

FIGS. 19J-K illustrate use of SIFT points to recognize objects in images. As shown in FIG. 19J, a number of different images that include a particular object, referred to as images “P1” to “Pn” 1962 a-e, are acquired. In general, each of these different images 1962 a-e are taken from different angles and positions with respect to the object in order to capture various different ways that the object may appear in a two-dimensional image. A set of SIFT features, or keypoints, are determined for each image and placed into a database of SIFT keypoints 1962 f. Note that each feature or keypoint is characterized by the above-discussed SIFT feature attributes 1962 g as well as an attribute 1962 h corresponding to the particular image, or pose, from which the feature was extracted. Note that, in addition, the features or keypoints are selected for lying within or close to the particular object that is to be characterized in subsequent, input images.

FIG. 19K illustrates how an object may be identified within an input image. In FIG. 19K, the input image, containing a subimage of the object 1964 a is shown in the top right-hand corner of the figure. The database of SIFT features 1962 f is again represented in FIG. 19K in the top left-hand corner. In a first step, SIFT features are extracted from input image 1964 a and matched to SIFT features in the database, as represented in FIG. 19K by labeled arrow 1964 b and pseudocode 1964 c. This produces a set of matches 1964 d in which successive pairs of adjacent rows represent an extracted SIFT feature from the input image and the corresponding SIFT feature from the database. The matching, as indicated in pseudocode 1964 c, is based on a matching of the 128-element descriptors of the extracted SIFT features with the descriptors of the database-resident SIFT features. This is carried out using a nearest-neighbor method that finds the database SIFT feature with a descriptor closest, in high-dimensional distance, to the descriptor of a considered SIFT point extracted from the input image. However, in order to avoid spurious matching, a match occurs only when the ratio of distances for the best-matching descriptor and the next-best-matching descriptor is lower than a first threshold and the lowest distance is lower than a second threshold.

In a next step, represented by vertical arrow 1964 e, Hough-Transform-based clustering is carried out in order to cluster the matches by pose. Each match of an input-image feature to a database feature can be considered to be a vote for a particular x,y translation, scale, and orientation angle. Clusters of a maxima in the four-dimensional Hough space correspond to the most probable object poses. Then, a RANSAC-like selection technique, represented by horizontal arrow 1964 f, is employed to select the most likely pose for the object from the clusters produced by Hough-Transform-based clustering. The features in the selected cluster can then be used to identify the location of the object in the input image.

FIG. 20 illustrates a second type of feature detector, referred to as the “Histogram of Gradients” (“HoG”) feature detector. HoG features are computed using similar computations used for a portion of the SIFT-feature-extraction methods, discussed above. In the HoG method, an input image is decomposed into blocks, such as block 2002 in FIG. 20, each block consisting of multiple cells, such as cell 2004. For example, a block may include 16×16=256 pixels while each cell includes 8×8=64 pixels. The gradient magnitudes and orientations are computed for each pixel in a cell and are weighted by the distances between pixels and an overlying Gaussian surface, indicated in FIG. 20 by the dashed circle 2006, Just as in the SIFT-feature-extraction method, the weighted gradient magnitudes are used to construct a cell histogram 2008, with bins representing ranges of orientation angles, for each cell in a block. The heights of the histogram bars are then encoded in adjacent elements 2010 of a block feature vector 2012. The block feature vectors are then combined to generate an extracted image feature for an image.

FIGS. 21A-B illustrate a third type of feature detector, referred to as the “Linear Binary Patterns” (“LBP”) feature detector. As shown in FIG. 21A, and eight-bit numeric value, or bit vector 2102 is generated for a currently considered pixel 2104 within a neighborhood of pixels 2106 within a digital image. Each bit in the eight-bit numeric value 2102 is generated from a comparison of the currently considered pixel with a neighboring pixel. For example, bit 2108 is generated from a comparison of the currently considered pixel 2104 with neighbor pixel 2110. When the currently considered pixel has a value greater than that of the neighboring pixel, as is the case for pixels 2104 and 2110, the bit value generated by the comparison is “0.” Otherwise, a bit value “1” is generated. Each bit in the eight-bit numeric value 2102 is generated from a comparison of the currently considered pixel with a unique neighboring pixel, as can be seen in FIG. 21A from the curved-arrows, such as curved arrow 2112, each indicating a correspondence between the comparison of the currently considered pixel with a different neighboring pixel. As shown in FIG. 21B, an image is partitioned into cells, such as 16×16 cell 2120, and an LBP eight-bit numeric value is computed for each pixel in the cell, using, for edge pixels, replicated cells or pixels from neighboring cells. The eight-bit numeric values are then used to create a cell histogram with 256 bins corresponding to the 256 possible numeric values for an integer represented by eight bits. The cell histogram can be alternatively considered to be a 256-element cell feature vector, and the cell feature vectors are then combined to generate an extracted image feature for an image.

The Methods and Subsystems to which the Current Document is Directed

FIG. 22 illustrates use of feature detectors in the methods and systems to which the current document is directed. Each of various different types of feature detector 2202-2204, which may include the previously discussed SIFT, HoG, and LBP feature detectors as well as other feature detectors, are separated applied to an image 2202, generally a normalized face-containing subimage of an input image, to generate a corresponding set of feature vectors 2208-2210, each set including one feature vector for each portion of the image, as defined by the particular feature detector, such as a cell, block, or other such portion. For some feature detectors, the portions are distinct while, for others, the portions are overlapping. The sets of feature vectors and be alternatively considered to be a two-dimensional, transformed image 2212-2214 by arranging the feature vectors as columns or rows within a two-dimensional matrix. These two-dimensional, transformed images may be linearized to form one-dimensional vectors 2216-2218. These one-dimensional vectors can be input into each of many different classifiers to generate attribute values for the human face contained in the image 2206.

FIGS. 23A-B illustrate a type of classifier referred to as a support vector machine (“SVM”). In general, a classifier receives input data and returns a value that represents a characteristic of the data. In the currently disclosed methods and systems, the one-dimensional vectors produced by feature detectors in response to input subimages can be considered to be data points in a very high-dimensional space, just as a vector of three coordinates in the x, y, and z directions of a Cartesian volume can be considered to specify a point in three-dimensional space. A binary classifier produces one of two possible output values, such as {0, 1}, {male, female}, {white, non-white}, or {true, false}. An SVM is generally trained, using training input data points for which desired output values are known, to partition a data-point space into two regions. Following training, the SVM, upon input of a data point with an unknown output value, determines in which of the two partitions of the data-point space the input data point is located and returns the output value associated with the partition of the data-point space in which the input data point is located. In FIG. 23A, example one-dimensional 2302, two-dimensional 2303, and three-dimensional 2304 SVMs are illustrated. In each example SVM, data points in a first partition are represented by filled disks, such as filled disk 2306, and data points in a second partition are represented by unfilled disks, such as unfilled disk 2308. In the one-dimensional SVM 2302, the horizontal line 2310 representing the data-point space is partitioned by a point on the line 2312 into a first, left-hand region 2314 and a second right-hand region 2316. In the two-dimensional SVM 2303, the plane 2320 representing the data-point space is partitioned by a line 2312 into a first region 2324 and a second region 2326. In the three-dimensional SVM 2304, the volume 2330 representing the data-point space is partitioned by a plane 2332 into a first region 2334 and a second region 2336. In these examples, each SVM classifier receives an input data point x and returns one of the two values {true, false} 2338.

FIG. 23B illustrates linear and non-linear SVMs. In a linear SVM 2340, the partition 2342 is an (n−1)-dimensional object within an n-dimensional data-point space. The partition can therefore be described by the expression 2344: w·x+b=0,

where w is a vector normal to the partition,

-   -   x is a data point on or within the partition, and     -   b is a constant.         The value

$\frac{- b}{w}$ is the shortest distance 2346 from the origin 2348 to the partition 2342. There are two additional partition-like elements 2350 and 2352 on either side of the partition 2342 with equations: w·x+b=1, and w·x+b=−1. The shortest distance between the partition and the additional partition-like elements 2350 and 2352 is |w|, the magnitude of vector w. The SVM is constructed by determining an equation for the partition that correctly partitions the two different sets of data points and that minimizes |w|, as an optimization problem. A non-linear SVM 2356 can be generated by replacing the dot-product operation with a function k( ): w·x→k(w,x), which is equivalent to a vector-space transform ϕ w*=ϕ(w) x*=ϕ(x) that transforms vectors in an original vector space S to a transformed vector space S*. The same optimization method can be used to generate a linear partition in the transformed vector space which is generally a curved partition in the original vector space.

FIG. 24 illustrates two additional, higher-level feature detectors used in the methods and systems to which the current document is directed. In FIG. 24, an initial input image 2402 is shown with a superimposed Cartesian two-dimensional coordinate system. Often, in image processing, the origin is assigned to the top, left corner of the image, with the y axis descending downward along the left edge of the image. However, any coordinate system may be used, including the traditional coordinate system 2404 with y and x axes coincident with the left and bottom image edges and the origin at the left, bottom corner. A face detector 2406 receives the input image 2402 and returns the coordinates 2408 for a corner 2410 of each rectangular subimage 2412 that frames a face 2413 in the input image. In addition, the face detector returns the height h 2414 and width w 2416 of each face-containing subimage. Face detectors generally use a variety of feature-extraction methods and classifiers, and may additionally use rules and other logic. A face-subregions detector 2418 receives, as input, an original input image 2402 and the parameters 2408, 2414, and 1416 that describe a face-containing subimage 2412 and returns parameters for rectangular sub-subimages 2420-2423, or regions, that contain, respectively, an image of the left eye, right eye, nose, and mouth. Separate face region detectors may be used rather than an aggregate face-regions detector. The parameters that specify the location and dimensions of regions and subimages may also differ for different detectors. For example, the coordinates for the top left and lower right corners of a subimage, sub-subimage, or region also completely specify the location and dimensions of the subimage, sub-subimage, or region.

FIG. 25 illustrates normalization of the regions obtained by application of a face detector and face-subregions detector, discussed above with reference to FIG. 24. As shown in the original input image 2502 in FIG. 25, the face region 2504 may be oriented differently than the input-image edges 2506-2509. In addition, the relative positions and orientations of the eye, nose, and mouth regions 2510-1523 may not coincide with expected positions and orientations, based on analysis of many human-face images. Therefore, rotation-matrix and translation-matrix operations, discussed above, may be performed to reorient the face region 2504 and eye, nose, and mouth regions 2510-1523 to produce a canonical region arrangement 2516. In certain cases, perspective transformations may also be employed to normalized regions containing perspective-distorted images. In cases in which the arrangement and orientations of the regions cannot be brought into conformance with a range of arrangements and orientations expected of human faces, the face-containing subimage may be rejected and removed from further consideration. Finally, as indicated by arrows 2518 and rectangles 2514-2528, the regions in the canonical region arrangement 2516 are extracted and rescaled, when necessary, to produce face, left-eye, right-eye, node, and mouth regions of the same size, in on implementation 114×144 pixels. Of course, in alternative implementations, fewer regions or a greater number of regions may be generated for each face image in an input image, using fewer or more feature detectors. In addition, it is often the case that the input image is initially processed to normalize the input image with respect to dimensions, contrast, and digital representation, which may involve rescaling and color-model transformations. In certain cases, multiple versions of input images are generated to provide the type of image needed by each different feature detector. Some feature detectors, for example, may use binary or grayscale images, while others may use color images.

FIG. 26 illustrates attribute classifiers employed in the methods and systems to which the current application is directed. As discussed above, with reference to FIG. 22, various type of feature-extraction methods, including SIFT, HoG, and LBP, are applied to the normalized regions, discussed above with reference to FIG. 26, to generate feature vectors for the regions, one-dimensional transformed images. These feature vectors 2606 are input to each of a number of attribute classifiers 2604, 2606, 2608, and 2610. Attribute classifiers may be single SVMs or other such classifiers, such as binary classifier 2604 that returns an indication of whether or not the region corresponding to the submitted feature vector corresponds to a male face or a female face. Alternatively, an attribute classifier may include multiple individual classifiers, such as classifier 2606, which includes three binary classifiers 2612-2614. In the case of aggregate attribute classifier 2606, the three composing classifiers all return output values indicating whether the region corresponding to a submitted feature vector corresponds to a male face or a female face. In this case, the output of the aggregate attribute classifier is computed from the outputs of the composing, individual classifiers, further discussed below. An aggregate attribute classifier may include multiple different composing classifiers, such as classifiers 2616-2620 within aggregate attribute classifier 2608. In this case, the aggregate attribute classifier returns an indication of the ethnicity of the person from whose image the submitted feature vector was extracted. Each composing classifier, such as classifier 2616, returns a value indicating whether or not the input feature vector was computed from an image of a particular ethnicity type. Similarly, aggregate attribute classifier 2610 returns an indication of the age range of the person whose image generated the input feature vector, while the individual composing classifiers 2622-2627 return a value indicating whether or not the age of the person is within a particular age range.

FIG. 27 illustrates the high-level architecture for the attribute-assignment image-processing system to which the current document is directed. A number a of normalized regions 2702-2706 are produced by the face and face-subregions detectors, discussed above with referenced to FIGS. 24-25, for each face-containing subimage of an input image. Feature vectors 2708 are produced by each of b feature detectors 2710-2716, discussed above with reference to FIGS. 19A-21B. Each feature detector produces a generally different feature vector for each of the normalized regions 2702-2706. Thus, in general, a×b feature vectors are produced from a face-containing subimage of an input image. These feature vectors are then distributed among the individual attribute classifiers and individual classifiers within aggregate attribute classifiers 2720-2726 which produce output attribute values that are collected in an output attribute-value vector 2728. Because the attribute-assignment image-processing system is generally implemented in a distributed computer system, the feature-extraction and classification tasks may be carried out in parallel over many different servers and processor cores.

FIGS. 28A-D illustrate how aggregate classifiers produce output values and associated probabilities. FIG. 28A illustrates generation of a discrete probability density function for internal-classifier outputs within an aggregate classifier using a set of input feature vectors from regions with a known attribute value. For example, feature vectors computed from regions containing male facial components may be input to the individual classifiers within a male/female aggregate classifier. When a feature vector 2802 is input to the aggregate classifier, the outputs from the individual composing classifiers, shown as an array 2804 of classifiers, are collected 2806. In FIG. 28A, the outputs are either “0” or “1,” and the input feature vector is computed from a region with attribute value “1.” The total number of correct, or positive, output values 2808 is then determined, and a corresponding bin 2810 in a histogram of total-positive results 2812 is incremented. After input of 539 different feature vectors for regions with attribute “1,” a discrete probability density function 2814 is generated, in the example of FIG. 28A.

As shown in FIG. 28B, a probability distribution function is easily computed from the probability density function 2814. In FIG. 28B, the probability distribution function is shown as a curve in which discrete values are connected by small line segments. In fact, the probability distribution function is discrete and is accurately represented as a set of points, rather than as a curve. Nonetheless, the continuous approximation shown in FIG. 28B is perhaps more readily understood. The meaning of the probability distribution function is that, for a given value along the horizontal axis 2822, such as point 2824 representing 20 individual classifiers, the percentage of the input feature vectors for which 20 or fewer individual classifiers output a positive or correct attribute value, in the current example, “1,” is given by the height of a vertical line segment 2826 passing through the given point along the horizontal axis 2824 and ending at a point 2828 on the probability-distribution curve 2830. This percentage, expressed as a fractional value in the range [0, 1], can be considered to be an estimate of the probability that, when a feature vector computed from a region with attribute value “1,” 20 or fewer individual classifiers will output the attribute value “1.” The first probability-distribution-curve point 2832 is computed as the number of counts in the first histogram bin 2833, 1, divided by the total number of experiments, or submitted feature vectors 539. This is the percentage of experiments in which no component classifier returned a “1” response. The second probability-distribution-curve point 2834 is computed as the number of counts in the first two histogram bins 2833 and 2835, 1+2=3, divided by 539. The accumulated counts for each bin and all preceding bins are shown below the classifier-positive-result-count values on the horizontal axis of the probability density function 2814.

In FIG. 28C, a probability-distribution curve 2836, plotted in a first plot 2837, is generated from many experiments in which feature vectors computed from male regions, or regions with a gender attribute assigned the value m, are submitted to a male/female aggregate classifier containing 22 individual classifiers. A second probability-distribution curve 2838, plotted in a second plot 2839, is generated from many experiments in which feature vectors computed from female regions are submitted to the male/female aggregate classifier and the number of f attribute-value outputting individual classifiers for each input feature vector are tabulated in a histogram. These two curves are positive response curves—one for male input feature vectors and the other for female input vectors. A negative response curve 2840 for the female experiments is obtained by subtracting each curve point of the positive curve 2838 from 1. In other words, the positive-result probability-distribution curve 2838 gives an estimate of the probability of up to a selected number of component classifiers returning a positive result and the negative-result probability-distribution curve 2840 gives an estimate of the probability of up to a selected number of component classifiers returning a negative result.

The negative-result probability-distribution curve 2840 is imported into the first plot 2837 in FIG. 28C, so that the first plot 2837 contains both the negative-result probability-distribution curve 2840 obtained from female-region experiments and the positive probability-distribution curve 2836 obtained from male-region experiments. There is a point 2842 where the two curves cross, referred to as the “crossover point.” A vertical line segment 2844 through that point crosses the horizontal axis at point 2846, which indicates the number of component classifiers, eight in the current example, returning the attribute value m when the input feature vector has an equal probability of having been computed from either a male or female region. When eight component classifiers return the value m, the probability that the region from which the input feature vector was generated is male is therefore 0.5. A probability table 2850 can be generated from the both the positive probability-distribution curve 2836 and the negative-result probability-distribution curve 2840. The probability of the region from which an input feature vector was generated being a male region when 13 (2852 in FIG. 28C) return the attribute value m is equal to 0.5+one half the vertical distance d′ 2854 between the intersection points of the two curves and the vertical line, 0.86 (2856 in FIG. 28C). For a point 2857 to the left of the cross-over point, one half the vertical distance d 2858 between the intersection points of the two curves and s vertical line through the point 2857 is subtracted from 0.5 to produce the result (2860 in FIG. 28C).

As shown in FIG. 28D, a similar approach is used to generate probability tables 2862 for aggregate classifiers that return one of multiple possible attribute values, such as the ethnicity aggregate classifier 2608 discussed above with reference to FIG. 26. In this case, multiple plots of positive and negative probability-distribution curves 2866-2870 are used to each generate a two-dimensional table, from which the probabilities 2872 of the ethnicity being each of the possible ethnicities can be computed from results output by the component classifiers. In one approach 2874, the most probable value is selected as the attribute value, when the difference in probability between the top two most probable attribute values is greater than a threshold difference. Otherwise, a no-decision result is returned. In other approaches, the most probably attribute value may be returned.

FIG. 29 illustrates a confusion matrix. The confusion matrix is obtained by observing the attribute values returned by a classifier for a number of input feature vectors with known attribute values. The confusion matrix 2902 shown in FIG. 29 is a hypothetical confusion matrix for the ethnicity aggregate classifier 2608 discussed above with reference to FIG. 26. Each column, such as the first column 2904, represents a number of experiments in which feature vectors with a known attribute value, indicated above the column, such as the symbol “w” 2906 above column 2904, are input to the aggregate classifier and results collected. Each cell in the column, such as cell 2908, includes the number of input feature vectors with the known attribute corresponding to the column that returned the attribute value associated with the row in which the cell is located. Cell 2908 indicates that of 6025 feature vectors with known attribute w submitted to the aggregate classifier, where 6025 is the total number of counts in the first column 2904, the aggregate classifier returned the result w 3662 times. The diagonal dells in the confusion matrix represent correct classifications by the aggregate classifier and the off-diagonal cells represent incorrect classifications. Thus, a figure of merit (“FOM”) can be computed for the full confusion matrix as:

${FOM}_{total} = {1 - \frac{\sum\limits_{i \neq j}{count}_{i,j}}{\sum\limits_{i,j}{count}_{i,j}}}$ or for a single column as:

${FOM}_{j} = {1 - {\frac{\sum\limits_{i \neq j}{count}_{i}}{\sum\limits_{i}{count}_{i}}.}}$ The FOM ranges over the interval [0,1] with the value 1 representing the best possible confusion matrix. The confusion matrices and corresponding FOMs corresponding to different collections of individual component classifiers can be used, in an optimization method, to select the component classifiers for any given aggregate classifier in order to achieve an aggregate classifier with the maximum possible FOM.

FIG. 30 illustrates the high-level operation of the attribute-assigning image-processing system to which the current document is directed. An input image containing subimages of one or more human faces 3002 is input to the processing system 3004, and the processing system, in response, returns descriptors 3006 and 2008 for each face-containing subimage in the input image. Each descriptor, such as descriptor 3006, may include a name and/or identifier for the subimage 3010, values 3011 and 3012 that specify the location and size of the subimage, and attribute values assigned to each of a number of face-containing-subimage attributes 3013-3016, such as gender, ethnicity, and age. Many additional types of attributes may be assigned values by different implementations of the attribute-assigning image-processing system.

FIG. 31 illustrates one physical implementation of the attribute-assigning image-processing system to which the current document is directed. The attribute-assigning image-processing system is implemented within a distributed computer system 3104, described in the first subsection of this document. Components of the attribute-assigning image-processing system include an input memory for storing input images 3106, and output memory in which descriptors of face-containing subimages are stored 3108, a set of first-level feature detectors 3110, such as the feature detectors discussed above with reference to FIGS. 24-25, a set of second-level feature detectors, such as feature detectors 2710-2716 discussed above with reference to FIG. 27, a set of attribute classifiers 3112, such as attribute classifiers 2720-2726 discussed above with reference to FIG. 27, and a controller 3114 that controls operations of the feature detectors and classifiers.

FIGS. 32-34 illustrate an efficient attribute-assignment method used in many implementations of the attribute-assigning image-processing system to which the current document is directed. In the discussion of this method, an example attribute-assignment problem is used in which the attribute-assigning image-processing system assigns values for the three attributes a1, a2, and a3 to each face-containing subimage in an input image. The full attribute-value space 3202 is represented in FIG. 32 by a Cartesian volume defined by mutually orthogonal axes 3204-3206 corresponding to attributes a1, a2, and a3, respectively. Attribute a1 can be assigned one of four values a, b, c, and d. Attribute a2 can be assigned one of three values i, j, and k. Attribute a3 can be assigned one of two values m and n.

The efficient method for attribute assignment orders the attributes a1, a2, and a3 by the reliability and robustness of the attribute classifiers used to determine values for the attributes. In the example of FIGS. 32-34, the order is: a3>a1>a2. The order can be determined from the FOMs for the attribute classifiers or by other means. Because the attribute classifier for attribute a3 is most reliable in classifying subimages, that classifier is first applied 3208 to greatly reduce the volume of the attribute-value space. The remaining attribute-value space 3210 is half the volume of the initial attribute-value space 3202. Because attribute classifiers return a probability along with the determined attribute value, whether or not the first-applied attribute classifier is effective in reducing the size of the attribute-value space can be ascertained from the probability associated with the returned attribute value. Given that the attribute assignment made by the first-applied attribute classifier appears reliable, then the next most reliable attribute classifier, in the current example, the attribute classifier for attribute a2, is next applied 3212. However, a modified attribute classifier is applied, when available. The modified attribute classifier is trained to return a value for attribute a2 assuming that attribute a1 has the value assigned by the first-applied attribute classifier. By using a more specific training, the reliability of a conditional classifier is often greater than that of an unconditional classifier. In other words, the task carried out by a conditional classifier is generally simpler, because the attribute-value space operated on by a conditional classifier is smaller than the attribute-value space remaining after successful previous application of one or more other attribute classifiers. Thus, the modified attribute classifier is trained on subimages with a known value for attribute a1. This type of attribute classifier is referred to as a conditional attribute classifier, since the conditional attribute classifier returns an attribute value with a conditional probability based on the value of the attribute previously assigned by the first-applied attribute classifier. When the second attribute assignment is associated with an acceptable probability, a third conditional attribute classifier for attribute a2 is then applied 3214 to assign a final value to the final attribute. This conditional attribute classifier returns an assignment of value to attribute a2 assuming that the previous assignments of values to attributes a3 and a1 are correct. At this point, a single set of values for all three attributes 3216 has been determined.

FIG. 33 illustrates the various possible unconditional and conditional attribute classifiers that may be developed for the example attribute-value space discussed above with reference to FIG. 32. The attribute-value space 3202 is shown again at the top of the figure. Three columns labeled a1 3302, a2 3303, and a3 3304 show the possible unconditional and conditional attribute classifiers for the three attributes a1, a2, and a3. In general, the reliability order for the unconditional classifiers may be determined, so that only a subset of the unconditional classifiers are need. However, various additional attribute classifiers may be developed for handling different types of input images in various implementations of the currently disclosed attribute-assigning image-processing system. Note that the various attribute classifiers are labeled with identifying integers, in parentheses.

FIG. 34 illustrates, using a possible-search-path-like tree, the method for efficient attribute assignment discussed above with reference to FIG. 32. As indicated at the top of FIG. 34, 3402, the attribute order a3>a1>a2 is assumed and attribute classifiers 28, 5, 16, 20, 21, 22, 23, 24, 25, 26, and 27, discussed above with reference to FIG. 33, are available. The root node 3404 represents the beginning of attribute assignment. Node 3406 represents application of the unconditional attribute classifier for attribute a3. The three child nodes 3408-3410 of node 3406 represent the possible outcomes of the application of the unconditional attribute classifier for attribute a3. When the unconditional attribute classifier for attribute a3 returns the attribute value m with a sufficiently high associated probability, as represented by node 3408, then a conditional attribute classifier for attribute a1 is applied, assuming a3=m, as represented by node 3412, and further child nodes 3414-1418 represent the possible outcomes. When the unconditional attribute classifier for attribute a3 returns the attribute value n with a sufficiently high associated probability, as represented by node 3409, then a conditional attribute classifier for attribute a1 is applied, assuming a3=n, as represented by node 3420, and further child nodes, such as child node 3422, again represent the possible outcomes. When the unconditional attribute classifier for attribute a3 cannot choose between attribute values m and n with a sufficiently high associated probability, as represented by node 3410, then the attribute-assignment path bifurcates, at node 3410, with two conditional attribute classifiers for attribute a1 applied, as represented by nodes 3424 and 3426, and further child nodes, such as child nodes 3428 and 3430, again represent the possible outcomes. The attribute assignments made in each of the assignment paths represented by nodes 3424 and 3426, and lower nodes, are compared, once completed, and the attribute assignments with highest probability are selected. Note that, should the remaining attribute values for attributes a1 and a2 be assigned in one or both of the branching assignment paths represented by nodes 3424 and 3426, and if the conditional attribute classifier for attribute a3 appropriate for those assignments available (one of attribute classifiers 7-12 in FIG. 33), then that conditional attribute classifier for attribute a3 can be applied to determine whether, with the additional attribute assignments now made, whether attribute a3 can now be assigned a value with sufficient probability.

FIGS. 35A-D illustrate one implementation of controller 3114 discussed with reference to FIG. 31. This controller employs the attribute-assignment method discussed above with reference to FIGS. 32-34. FIG. 35A provides a control-flow diagram for the controller. In step 3502, an input image is received and preprocessed, as discussed above, to normalize the image with respect to digital representation, scale, contrast, and other factors. In step 3504, the face-region detector is applied, as discussed above with reference to FIG. 24, to generate a face-containing subimage for each face image in the input image. In the outer for-loop of steps 3506-3513, each identified face subimage is processed. In step 3507, normalized regions are generated for the currently considered face subimage by applying the face-subregions detector and normalization operations, as discussed above with reference to FIGS. 24-25. In the inner for-loop of steps 3508-3510, second-level feature extractors are applied to the normalized regions, as discussed above with reference to FIG. 27. In step 3511, the recursive routine “determine attributes” is called to apply attribute classifiers, as discussed above with reference to FIGS. 27 and 32-34, to assign attribute values to the attributes for the currently considered face-containing subimage. In step 3512, a descriptor is generated for the currently considered face-containing subimage, as discussed above with reference to FIG. 30. When the outer for-loop of steps 3506-3513 terminates, descriptors are produced for the identified face-containing subimages.

FIG. 35B illustrates certain data structures used by the recursive routine “determine attributes.” An array ordered_attributes 3514 includes indexes into an attributes data structure 3516. The order of the indexes reflects the order in which attribute classifiers for the indexed attributes are applied, as discussed above with reference to FIGS. 32-34. The attributes data structure 3516 includes descriptors for the attributes, each descriptor including a reference, such as reference 3517, to a list of attribute classifiers available for the attribute, as discussed above with reference to FIG. 33. The two-dimensional array attribute values 3522 includes indications of the attributes and associated probabilities assigned to attributes. The variable numAttributes includes a value that represents the number of attributes for which assignments are made.

FIGS. 35C-D provide a control-flow diagram for the recursive routine “determine attributes,” called in step 3511 of FIG. 35A. In step 3530, the routine “determine attributes” receives the data structures discussed above with reference to FIG. 35B, the feature vectors produced by the second-level feature detectors, in step 3509 of FIG. 35A, an a variable cA that indicates which attribute that needs to be next assigned a value. When the value in variable cA is greater or equal to the value in numAttributes, as determined in step 3532, all attribute values are assigned, in which case the current instance of the routine “determine attributes” returns. Otherwise, in step 3533, the variable c is set to reference the list of classifiers for the currently considered attribute. In step 3534, a classifier reference by the variable class is selected. This classifier is the classifier that is best suited for application to the feature vectors to determine a value for the currently considered attribute, based on the values already assigned to attributes, if any. In step 3535, an attribute value a and an associated probability p are generated by applying the classifier reference by the reference stored in the variable class. When the classifier returns a definite attribute value a with an associated probability p greater than a threshold value, as determined in step 3536, the returned values a and p are placed into the array attribute_values and cA is incremented, in step 3537, and the routine “determine attributes” is recursively called in step 3538. Otherwise, an attribute-assignment-path bifurcation occurs, and control flows to step 3539 of FIG. 35D.

In step 3539, an array of attribute_value arrays, avs, is initialized. Each attribute_value array in avs is initialized to contain the current attribute assignments. Each attribute_value array in avs is used for a separate recursive call to the routine “determine attributes.” In the for-loop of steps 3541-3544, the routine “determine attributes” is recursively called for each possible attribute value of the currently considered attribute, indexed by the value in cA. Following termination of the for-loop of steps 3541-3544, the attribute_value array in avs with the best associated probabilities is selected as the best assignment of attributes following the currently considered attribute, in step 3545. When, based on the values assigned to the following the currently considered attribute, there is another, more specific classifier for the currently considered attribute, as determined in step 3547, then that classifier is applied, in steps 3548-3549 to the feature vectors. When the more specific classifier returns an attribute with an associated probability greater than a threshold value, as determined in step 3550, the returned attribute value and associated probability are entered into the array attribute_values in step 3552. Otherwise, an indication that a value for the currently considered attribute cannot be determined is entered into the array attribute_values in step 3551. When there is no additional classifier to apply, as determined in step 3547, an indication that a value for the currently considered attribute cannot be determined is entered into the array attribute_values in step 3553. Finally, the attribute values for attributes following the currently considered attribute are copied from the selected avs array into the array attribute_values, in step 3554.

Although the present invention has been described in terms of particular embodiments, it is not intended that the invention be limited to these embodiments. Modifications within the spirit of the invention will be apparent to those skilled in the art. For example, any of many different design and implementation parameters may be varied to produce alternative implementations of the currently disclosed methods and systems. These parameters may include hardware layers, operating systems, programming languages, modular organization, control structures, data structures, and other such parameters. It should be noted that the above described methods and systems can be more generally applied to many additional problem domains in which attributes are assigned to entities based on observed evidence. For example, multivariate classification may be carried out on manufactured parts on an assembly line to determine whether or not they should be accepted, returned for modification, or rejected. As another example, attributes may be assigned to web-site visitors or social-network participants based on the history of their interactions with the website or social network. The current method of dividing and conquering a large attribute-value space by rational ordering of attribute-classifier application, facilitated by attribute classifiers that return probabilities in addition to attributed values, greatly expands the range of complex problem domains that can be addressed by application of classifiers, including SVM classifiers.

It is appreciated that the previous description of the embodiments is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 

The invention claimed is:
 1. An attribute-assigning image-processing system comprising: a distributed computing system that includes multiple computer systems, each having one or more processors, one or more memories, one or more mass-storage devices, and one or more network interconnections; an input-image memory for receiving an input image provided by one or more of the multiple computer systems; a set of first-level feature detectors, provided by one or more of the multiple computer systems, that detect face-containing subimages within the input image and, for each face-containing subimage, generate a set of normalized regions; a set of second-level feature detectors that generate a set of feature vectors from each normalized region; a set of attribute classifiers that each outputs an attribute value and an associated probability that reflects a likelihood that the attribute value returned in response to input of one or more feature vectors reflects the attribute value that would be assigned to the image of the facial feature contained in the normalized region from which the set of second-level feature detectors produced the input feature vectors; and a controller that submits the input image to the first-level feature detectors in order to generate sets of normalized regions for each face-containing subimage in the input image; submits each set of normalized regions to generate a set of feature vectors; and submits subsets of feature vectors to each attribute classifier in an ordered list of attribute classifiers to assign attributes to each face-containing subimage.
 2. The attribute-assigning image-processing system of claim 1 wherein the set of first-level feature detectors comprises: a set of multiple face detectors that each receives a reference to the input image and returns parameters that specify a subimage that frames a human face in the input image; and a set of multiple face-subregions detectors that each receives a reference to the input image and a set of parameters that specify a face-containing subimage and that each returns a set of parameters that species a sub-subimage, or region, that contains an image of a facial feature.
 3. The attribute-assigning image-processing system of claim 1 wherein a set of normalized regions corresponding to a face-containing subimage is generated by one or more of: performing rotation-matrix and translation-matrix operations to reorient the face-containing subimage and the corresponding regions that each contains an image of a facial feature to produce a canonical region arrangement; and applying one or more perspective transformations to corresponding regions that each contains a perspective-distorted image of a facial feature.
 4. The attribute-assigning image-processing system of claim 1 wherein the set of second-level feature detectors that generate a set of feature vectors from each normalized region further comprise: multiple feature detectors, including feature detectors selected from one or more of scale-invariant-feature-transform (“SIFT”) feature detectors, histogram-of-gradients (“HoG”) feature detectors, and linear-binary-patterns (“LBP”) feature detectors.
 5. The attribute-assigning image-processing system of claim 1 wherein each feature vector in the set of feature vectors generated by the second-level feature detectors includes encodings of multiple numeric values.
 6. The attribute-assigning image-processing system of claim 1 wherein the set of second-level feature detectors includes a number a of feature detectors, where a is greater than 2, that each generates a feature detector for each of b normalized regions, where b is greater than or equal to 5, resulting in generation of ab feature vectors that are distributed among the attribute classifiers.
 7. The attribute-assigning image-processing system of claim 1 wherein the set of attribute classifiers includes: one or more individual support vector machine (“SVM”) classifiers that each returns, in response to input of one or more feature vectors, one of two possible attribute values; and one or more aggregate classifiers that each includes two or more individual support vector machine (“SVM”) classifiers and that each returns, in response to input of one or more feature vectors, one of multiple possible attribute values.
 8. The attribute-assigning image-processing system of claim 1, wherein the probability returned by an aggregate classifier that returns one of two attribute values x and y is computed from a first probability distribution generated from results of applying the aggregate classifier to a set of test normalized regions known to have the attribute value x and from a second probability distribution generated from results of applying the classifier to a set of test normalized regions known to have the attribute value y.
 9. The attribute-assigning image-processing system of claim 8 wherein the first probability distribution for the test normalized regions known to have the attribute value x is obtained from a tabulation of the number the number of component classifiers of the aggregate classifier returning the attribute value x for each test normalized regions to have the attribute value x and the second probability distribution for the test normalized regions known to have the attribute value y is obtained from a tabulation of the number the number of component classifiers of the aggregate classifier returning the attribute value y for each test normalized regions to have the attribute value y.
 10. The attribute-assigning image-processing system of claim 8 wherein the first and second probability distributions are considered to be first and second positive response curves and a negative response curve is generated from the second positive response curve; and wherein a point along an axis corresponding to the number of classifiers returning an attribute value where the first positive response curve intersects the negative response curve is considered to be a crossover point xp.
 11. The attribute-assigning image-processing system of claim 10 wherein the probability returned by the aggregate classifier that returns the attribute x for an input normalized region when n component classifiers return the attribute x is computed from the first positive response curve and the negative response curve by: when xp−x is greater than 0, returning 0.5−½v, where v is a vertical distance between intersection points of a vertical line passing through x with the first positive response curve and the negative response curve; when xp−x is greater than 0, returning 0.5+½v, where v is a vertical distance between intersection points of a vertical line passing through x with the first positive response curve and the negative response curve; and when xp−x is equal to 0, returning
 0. 12. The attribute-assigning image-processing system of claim 1, wherein the probability returned by an aggregate classifier that returns one of more than two attribute values is obtained from a two-dimensional table generated from multiple plots of positive and negative probability-distribution curves.
 13. The attribute-assigning image-processing system of claim 1 wherein the attribute values output by the set of attribute classifiers is collected in an output attribute-value vector corresponding to a face-containing subimage.
 14. The attribute-assigning image-processing system of claim 1 wherein each attribute classifier is assigned a reliability.
 15. The attribute-assigning image-processing system of claim 1 wherein the reliability assigned to an attribute classifier is obtained from a computed figure of merit.
 16. The attribute-assigning image-processing system of claim 1 wherein attribute classifiers are applied in order of decreasing reliability.
 17. The attribute-assigning image-processing system of claim 16 wherein, once an attribute classifier A returns an attribute value x with greater than a threshold probability, subsequently-applied attribute classifiers are conditional attribute classifiers trained on test normalized regions having the attribute x.
 18. The attribute-assigning image-processing system of claim 16 wherein attribute application follows one or more paths through an acyclic graph, or tree.
 19. The attribute-assigning image-processing system of claim 16 wherein a path bifurcates when application of an attribute classifier does not return a probability greater than a threshold probability. 